I’d guess the mangling has more to do with Mastodon not trying to render the “Article” type yet than it being from a group. Mastodon has the same problem with reviews from Bookwyrm and blog posts from WriteFreely.
I mean, at least it shows the title and link, which is better than the “this type isn’t supported yet” from Takahe, but with the size of its user base, Mastodon *really* needs to get on just treating an Article the same way it treats a long Note!